1. Field of the Invention
The present invention relates to a heating apparatus for a vehicle, in which a heat-generating unit using a shearing force, for increasing a temperature of cooling water for cooling a water-cooled engine, is disposed in a cooling water circuit.
2. Description of Related Art
Conventionally, as a heating apparatus for a vehicle, a hot water type heating apparatus for heating a passenger compartment has been generally known. In the hot water type heating apparatus, cooling water for cooling a water-cooled engine is supplied to a heater core disposed in a duct, and air heated while passing through the heater core is blown into the passenger compartment by a blower to heat the passenger compartment of the vehicle.
However, in a case of a vehicle in which heat generated in the engine is too small to heat the cooling water with engine sufficiently, such as a vehicle having a diesel engine or a lean burn engine, because a temperature of cooling water in a cooling water circuit cannot be maintained at a predetermined temperature (e.g., 80.degree. C.), there occurs a problem in that a heating capacity for the passenger compartment is insufficient.
To overcome such a problem, as disclosed in JP-A-6-92134, the following heating apparatus for a vehicle is conventionally proposed. In the heating apparatus for a vehicle, a heat-generating unit using a shearing force, for heating cooling water to be supplied to a heater core from an engine, is disposed in a cooling water circuit.
The heat-generating unit transmits a driving force of the engine to a shaft through a belt transmitting mechanism and the electromagnetic clutch. A heat-generating chamber is formed in a case of the heat-generating unit, and a cooling water passage is formed at an outer periphery of the heat-generating chamber. A rotor which rotates integrally with the shaft is disposed in the heat-generating chamber, and a shearing force generated by a rotation of the rotor is applied to viscous fluid such as silicon oil sealed in the heat-generating chamber to generate heat. The cooling water circulating in the cooling water passage is heated by the generated heat.
However, according to the above-described conventional heating apparatus for a vehicle, since the driving force (i.e., driving torque) acting in the shaft of the heat-generating unit is converted into a supplementary heating source, a large load due to the driving torque (i.e., driving load) is applied to the engine. Therefore, there is a problem in that fuel consumption rate of the engine lowers and running cost of the vehicle rises.
Further, because only the temperature of the cooling water determines whether or not the heat-generating unit is operated, an electromagnetic clutch may be turned on and the heat-generating unit may be driven when the temperature of the cooling water is less than a set temperature at warm-up operation in the summer. Thus, the viscous fluid in the heat-generating chamber generates an abnormal heat, and heat-deterioration of the viscous fluid and mechanical deterioration due to a shearing force may be generated.
To prevent the heat-generating unit from being operated excessively, the set temperature of the cooling water may be decreased so that deterioration of the fuel consumption rate can be suppressed. However, in this case, because the temperature of the cooling water to be supplied to a heater core cannot be maintained at a predetermined temperature, a radiating amount of the heater core is decreased. Therefore, there occurs a problem in that a sufficient heating capacity cannot be obtained when a passenger in the passenger compartment desires the maximum heating operation.